It is known in the art to use inks, compositions, coatings or layers containing oriented magnetic or magnetisable pigment particles, particularly also optically variable magnetic or magnetisable pigment particles, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetisable pigment particles are disclosed for example in U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,676,273; U.S. Pat. No. 3,791,864; U.S. Pat. No. 5,630,877 and U.S. Pat. No. 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features on the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a document or item secured therewith, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
Magnetic or magnetisable pigment particles in printing inks or coatings allow for the production of magnetically induced images, designs and/or patterns through the application of a corresponding magnetic field, causing a local orientation of the magnetic or magnetisable pigment particles in the unhardened coating, followed by hardening the latter. The result is a fixed magnetically induced image, design or pattern. Materials and technologies for the orientation of magnetic or magnetisable pigment particles in coating compositions have been disclosed in U.S. Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864, DE 2006848-A, U.S. Pat. No. 3,676,273, U.S. Pat. No. 5,364,689, U.S. Pat. No. 6,103,361, EP 0 406 667 61; US 2002/0160194; US 2004/70062297; US 2004/0009308; EP 0 710 508 A1; WO 2002/09002 A2; WO 2003/000801 A2; WO 2005/002866 A1; WO 2006/061301 A1; these documents are incorporated herein by reference. In such a way, magnetically induced patterns which are highly resistant to counterfeit can be produced. The security element in question can only be produced by having access to both, the magnetic or magnetisable pigment particles or the corresponding ink, and the particular technology employed to print said ink and to orient said pigment in the printed ink.
The literature, such as for example in “Special Effect Pigments”, G. Pfaff, 2nd Revised Edition, 2008, pages 43 and 116-117, teaches that large reflective particles are preferred for producing images, designs or patterns because they have a large smooth surface, exhibit a uniform reflection of incident light thus leading to excellent lustre and brilliance, whereas small particles exhibit an increased light scattering and refraction thus causing reduced light reflection and inferior brilliance. Furthermore, it is known in the art that the qualities expressed by saturation, brightness, opacity of inks or compositions are affected by the size of the so-comprised pigment particles. For example, large optical effect pigment particles exhibit a higher chroma than corresponding smaller pigment particles. Therefore, the man skilled in the art typically uses reflective pigment particles having a large size, in particular optically variable pigment particles or optically variable magnetic or magnetisable pigment particles for producing optical effect layers. For example, the prior art discloses particles with an individual particle size lying in a range between 2 and 200 μm (microns). WO 2002/073250 A1 discloses optically variable magnetic or magnetisable pigment particles having a size between 20 and 30 μm. WO 2007/131833 A1 discloses coating compositions for producing magnetically induced images. It is further disclosed that, with the aim of getting saturated colours and abrupt colour changes, suitable optically variable magnetic or magnetisable pigment particles are characterized by a mean particle diameter (d50) typically ranging from 5 to 40 μm and preferably from 15 to 25 μm. WO 2011/012520 A2 discloses flake-shaped particles having a diameter of typically between 10 to 50 μm. WO 2006/061301 A1 discloses that a large particle size (flake diameter in the range of 10 to 50 μm) and a size distribution which is as homogeneous as possible, are desirable, in order to yield the optimum effect. U.S. Pat. No. 6,818,299 discloses pigment flakes having a dimension on any surface thereof ranging from 2 μm to 200 μm. US 2012/0107738 discloses flakes which are typically 5 μm to 100 μm across, more typically 20 μm to 40 μm across.
U.S. Pat. No. 8,025,952 discloses that the typical size of magnetic particles for inks is in the range of from 10 μm to 100 μm, more preferably in the range of from 18 to 30 μm. EP 2 402 401 A1 discloses pigment flakes having an average particle size between 2 and 20 μm.
Examples of dynamic security features based on magnetically induced images, designs or patterns providing the optical illusion of movement have been developed including without limitation rolling-bar effects and moving rings effects.
For example, U.S. Pat. No. 7,047,883 discloses the creation of a dynamic optically variable effect known as the “rolling bar” feature. The “rolling bar” feature provides the optical illusion of movement to images comprising oriented magnetic or magnetisable pigments. U.S. Pat. No. 7,517,578 and WO 2012/104098 A1 respectively disclose “double rolling bar” and “triple rolling bar” features, said features seeming to move against each other upon tilting. A printed “rolling bar” type image shows one or more contrasting bands which appear to move (“roll”) as the image is tilted with respect to the viewing angle. Such images are known to be easily recognized by the man on the street and the illusive aspect cannot be reproduced by commonly available office equipment for color scanning, printing and copying. “Rolling bar” features are based on a specific orientation of magnetic or magnetisable pigments. In particular, the magnetic or magnetisable pigments are aligned in a curving fashion, either following a convex curvature (also referred in the art as negative curved orientation) or a concave curvature (also referred in the art as positive curved orientation). This effect is nowadays utilized for a number of security elements on banknotes, such as the “5” of the 5 Euro banknote or the “100” of the 100 Rand banknote of South Africa.
For example, U.S. Pat. No. 8,343,615, EP 0 232 567 07 A2, WO 2011/092502 and US 2013/0084411 disclose moving-ring images displaying an apparently moving ring with changing viewing angle (“rolling ring” or “moving ring” effect).
As taught by the prior art, optically reflective non-spherical pigment particles having a large size, in particular optically variable non-spherical pigment particles having a large size, have been widely preferred for producing optical effect layers. While there are only limited indications available in the art describing preferred particle sizes for reflective non-spherical magnetic or magnetisable pigment particles or optically variable non-spherical magnetic or magnetisable pigment particles, those indications also point towards large particle sizes to obtain magnetically oriented optical effect layers with high reflectivity, chroma and/or colorshifting properties.
However, the present inventors found that, in practice, the optical properties of such optical effect layers may suffer from the use of particles with inadequately selected sizes, particularly from sizes that are excessively large, even though these sizes were until now considered as fine to very fine. Moreover, the use of large size particles was found to have also some disadvantages as they require specific printing elements such as special screens for the screen printing process or engraved cylinders for flexography and rotogravure. In particular, the fact that the size of the mesh for the screen or the dimension of the engraved structures of the engraved cylinders must be large enough to be compatible with large particles, induces an increase of the thickness of the OEL with associated drawbacks of a reduced drying speed combined with an increased ink consumption.
Therefore, a need remains for optical effect layers based on magnetically oriented pigments particles and displaying an eye-catching dynamic effect, to provide, in combination, a high resolution and a high contrast with equivalent or improved reflectivity.